To provide cellular wireless communication service, a wireless service provider typically operates a radio access network (RAN) that defines one or more wireless coverage areas, in which mobile stations can be served by the RAN and can thereby communicate with other mobile stations and obtain connectivity with broader networks such as the public switched telephone network (PSTN) and the Internet.
A typical RAN may include one or more base transceiver stations (BTSs) (e.g., macro network cell towers or femtocells), each of which may radiate to define one or more wireless coverage areas such as cells and cell sectors in which wireless mobile stations (MSs) can operate. “Sector” will be used hereinafter to refer generally to any wireless coverage area. Further, the RAN may include one or more base station controllers (BSCs), radio network controllers (RNCs) or the like, which may be integrated with or otherwise in communication with the BTSs and may include or be in communication with a switch or gateway that provides connectivity with one or more transport networks. Conveniently with this arrangement, a cell phone, personal digital assistant, wirelessly equipped computer, or other mobile station (whether or not actually operated by a user) that is positioned within coverage of the RAN can then communicate with a BTS and in turn, via the BTS, with other served devices or with other entities on the transport network.
In general, a RAN will communicate with served mobile stations according to an agreed air interface protocol, examples of which include CDMA (E.G., 1xRTT or 1xEV-DO), iDEN, WiMAX, LTE, TDMA, AMPS, GSM, GPRS, UMTS, or EDGE, and others now known or later developed. Communications in the direction from the RAN to mobile stations define a “forward link,” while those in the direction from mobile stations to the RAN define a “reverse link.”
A typical air interface protocol will provide a mechanism to distinguish communications in one sector from those in adjacent sectors and to distinguish between communications within a given sector. Under some air interface protocols, for instance, each sector may have a sector identifier that distinguishes the sector from adjacent sectors, and communications in the sector may designate or be encoded with that sector identifier in order to distinguish the communications from those in adjacent sectors. Likewise, each air interface connection (e.g., communication channel or other assigned connection resource) in a sector may have by a radio-link identifier, and communications carried on that connection may designate or be encoded with that radio-link identifier in order to distinguish the communications from others in the sector.
For example, under the CDMA 1xRTT protocol, each sector has a locally unique pseudonoise offset (“PN offset”) that is used to encode communications in the sector in a manner that distinguishes from communications in adjacent sectors, and each sector defines various control channels and traffic channels that are each encoded with a respective “Walsh code.” As another example, under the CDMA 1xEV-DO protocol, each sector similarly has a PN offset that distinguishes communications in the sector from those in adjacent sectors, and each sector designates connections assigned to various mobile stations by respective “MAC Indexes” (which may translate to Walsh codes similarly used to encode communications). Other examples are possible as well.
A RAN will typically broadcast a pilot signal respectively in each sector, to enable mobile stations to detect and evaluate cellular coverage. Further, the pilot signal of each sector may embody or designate the sector identifier, so that mobile stations can determine which sector is emitting the pilot signal. Under CDMA, for instance, the RAN may broadcast in each sector a pilot signal encoded with the sector's PN offset, so that if a mobile station detects a pilot signal encoded with that PN offset, the mobile station may determine that the PN offset is the sector identifier of the sector that is emitting the pilot signal.
The RAN may broadcast other status or informational messages on a sector-by-sector basis as well that a mobile station within or near a particular sector could receive. For instance, according to EV-DO protocol, a sector may track its reverse noise, that is, the degree of activity that the sector is receiving, and may set or clear a Reverse Activity Bit (RAB) depending upon whether the reverse noise is above or below a given baseline. A sector may determine its RAB once every 1.67 milliseconds, the frequency at which it may transmit forward-link timeslots, or at another frequency. After determination of the RAB, the sector may broadcast the RAB as a stand alone message or embedded within another message.
A sector may broadcast other status or informational messages to mobile stations within or near the sector. As one example, each sector may maintain a “neighbor list” that lists sectors neighboring (adjacent to or otherwise nearby), designating each neighboring sector by its sector identifier. The sector may broadcast neighbor list messages so that mobile stations could receive list the neighboring sectors or updates to the sector's neighbor list. As further examples, a sector may broadcast sector parameter messages, access parameter messages, or conflict messages to those mobile stations within or near the sector. A sequence or series of RAB or other status or informational messages may be described as a “signaling trend.”
In an “idle” or “dormant” state where a mobile station is not actively engaged in a call or other communication session, the mobile station may regularly monitor the strength (e.g., signal-to-noise ratio (SNR)) of various pilot signals in search of a strongest pilot signal and thus a best sector in which to operate. If and when the mobile station subsequently seeks to initiate a communication session, the mobile station may send a connection request (e.g., origination request) on an access channel of the selected sector, requesting the RAN to assign or otherwise establish a connection for the session. In response, the RAN may then assign a particular radio-link identifier (e.g., traffic channel or connection identifier, such as Walsh code or MAC Index) to the mobile station to be used in the sector, thereby transitioning the mobile station to an “active” state.
In the active state, when the mobile station is operating with an assigned connection in a given sector, the mobile station may regularly monitor the strength (e.g., SNR) of the pilot signal in that sector and the strengths of the pilot signals in neighboring sectors. If the pilot signal from another sector becomes sufficiently stronger than the pilot signal from the current serving sector (e.g., as a result of the mobile station moving toward the adjacent sector), the mobile station may then engage in control channel signaling with the RAN to arrange for a handoff of the communication session from the current sector to the other sector.
Under certain air interface protocols, such as CDMA for instance, a mobile station can operate actively in more than one sector at a time. Such an arrangement helps when the mobile station passes through an area of overlap between two or more sectors, as the mobile station may then engage in a “soft handoff” process that involves switching to communicate in a new sector before discontinuing communication in a previous sector. Further, soft handoff provides other advantages, such as allowing the mobile station or the RAN to combine together or select the best quality of communications carried out simultaneously in the multiple sectors.
To facilitate soft handoff, a mobile station may maintain in its memory an “active set” that lists the sectors in which the mobile station has an active connection, and the RAN may likewise maintain a record of the mobile station's active set and will communicate with the mobile station in each listed sector. The active set may designate each sector by its sector identifier and may further designate the connection assigned to the mobile station in that sector by its radio-link identifier. Generally, an active set may be limited in size to some defined number of sectors, such as three or six for instance.
In practice, the RAN may also provide the mobile station with a “neighbor list” that lists sectors neighboring (adjacent to or otherwise nearby) those in the mobile station's active set, designating each neighboring sector by its sector identifier. The mobile station may then regularly evaluate the strength of pilot signals emitted by each sector of its active set and the strength of pilot signals emitted by each sector listed in the neighbor list, as well as the strength of other (remaining) pilot signals that the mobile station detects even if not listed in the mobile station's active set or neighbor list. If the mobile station thereby detects a pilot signal that is sufficiently strong compared with the weakest of the mobile station's active set members, the mobile station may then engage in signaling with the RAN to arrange for soft handoff to the detected sector and perhaps removal of the weaker sector from the mobile station's active set.
When a mobile station begins a communication session, the mobile station's active set may consist of just the sector in which the mobile station sent its connection request. At that point, the BTS serving that sector may provide the mobile station with a neighbor list designating neighbors of that one sector. As the communication session proceeds, the mobile station may then detect other sectors and arrange for addition of those other sectors to its active set through soft handoff.
Alternatively, a session can be initiated through a process known as “channel assignment into soft handoff” (CASHO), where the mobile station begins the session in a soft handoff state (having multiple sectors in its active set) rather than transitioning to that state over time in the session. In the CASHO process, the mobile station identifies multiple candidate sectors to initially include in its active set and, when requesting a connection in a particular sector, the mobile station provides the serving BTS with a list of the identified sectors (each designated by sector identifier). The RAN may then establish a connection for the mobile station respectively in each identified sector, so that the mobile station can begin the communication session in a soft handoff state, active in multiple sectors at once.